Enhanced real-time ammonia slip detection

ABSTRACT

Various systems and methods are described for detecting ammonia slip. In one example method, an exhaust system with two NOx sensors uses transient responses of the NOx sensors to allocate tailpipe NOx sensor output to NOx and NH 3  levels therein. An ammonia slip detection counter with gains is included that determines a probability of NOx and NH 3  based on the measured sensor activities that are further processed by a controller to adjust one or more parameters based on the allocation and changes of sensor output.

FIELD

The present application relates generally to ammonia slip detection inan exhaust gas treatment system included in an exhaust system of aninternal combustion engine.

BACKGROUND AND SUMMARY

Diesel vehicles may be equipped with an exhaust gas treatment systemwhich may include, for example, a urea based selective catalyticreduction (SCR) system and one or more exhaust gas sensors such asnitrogen oxide (NO_(x)) sensors, at least one of which may be disposeddownstream of the SCR system. When the SCR system becomes loaded withurea to a point of saturation, which varies with temperature, the SCRsystem may begin to slip ammonia (NH₃). The NH₃ slip from the SCR systemmay be detected by the tailpipe NO_(x) sensor as NO_(x) resulting in aninaccurate NO_(x) output which is too high. As such, the efficiency ofthe SCR system may actually be higher than the efficiency determinedbased on the inaccurate NOx output.

US 2012/0085083 describes a method for estimating NOx conversion using apolynomial model that also allows for the NH₃ concentration at thedownstream tailpipe NOx sensor to be estimated. As described therein,temporal sensor signatures of a feedgas NOx sensor located upstream ofthe SCR and a tailpipe NOx sensor located downstream of the SCR arequantified and fit using a polynomial model that enables estimation ofNH₃ slip and NOx conversion efficiency. However, because the method usesa segment of each sensor signal for processing, a time lag existsbetween the acquisition of each NOx sensor output signal and theallocation of downstream NOx sensor output to NOx and NH₃. When the timelag is combined with the polynomial fitting algorithm described, whichmay be prone to localized estimation errors, realization of a real-timeNH₃ slip detection system by the methods described would be difficult toimplement.

The inventors have recognized disadvantages with the approach above andherein disclose methods for the real-time control of ammonia slip in anengine exhaust system. Methods described use transient responses of aNOx sensor to identify the rates of change of a NOx signal. Then, aprocessor further uses the rates of change to determine how thedownstream tailpipe NOx sensor is expected to change based on the flowupstream of the SCR, which allows allocation of a tailpipe NOx sensor inthe manner described below with substantially no perceivable delays inprocessing.

In one particular example, the exhaust system includes two NOx sensorsthat continuously monitor the exhaust gas flow upstream and downstreamof an SCR device. Then, when entry conditions of the engine system aremet, for example when the SCR device is above a temperature threshold,the rate of change of the upstream feedgas NOx sensor is combined with acurrent tailpipe reading to estimate the rate of change of the tailpipeNOx sensor expected based on the feedgas signal slope. The expectedtailpipe NOx signal is then compared with the actual NOx signal in orderto allocate the NOx sensor output to NOx and NH₃.

In another example, a method is provided that comprises allocating a NOxsensor output to each of NH₃ and NOx based on an upstream NOx rate ofchange and a downstream NOx rate of change relative to the SCR emissiondevice, which thereby allows the amount of reductant delivered to theengine exhaust to be adjusted based on the relative sensor signals.Because the method uses transient responses of the upstream anddownstream NOx sensors, in addition to the expected NOx signal, it istherefore possible to achieve a high level of NH₃ detection. In thisway, it is possible to provide enhanced allocation of the NOx sensoroutput in order to determine the relative NOx and NH₃ levels in theexhaust system.

The present description may provide several advantages. In particular,the approach may allow for the real-time detection of NH₃ slip with ahigh level of detection sensitivity without high feedgas NOxinterventions. Thus, NH₃ slip can be detected while a vehicle is inoperation and corrective measures taken based on the current state ofthe exhaust system. Furthermore, because the detection sensitivity isincreased, high levels of NOx are not required in order to determine theallocation of the tailpipe NOx sensor output to NOx and NH₃.

The above advantages and other advantages, and features of the presentdescription will be readily apparent from the following DetailedDescription when taken alone or in connection with the accompanyingdrawings. It should be understood that the summary above is provided tointroduce in simplified form a selection of concepts that are furtherdescribed in the detailed description. It is not meant to identify keyor essential features of the claimed subject matter, the scope of whichis defined uniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages described herein will be more fully understood by readingan example of an embodiment, referred to herein as the DetailedDescription, when taken alone or with reference to the drawings, where:

FIG. 1 shows a schematic diagram of an engine including an exhaustsystem with an exhaust gas treatment system.

FIGS. 2 A-D shows graphs illustrating an ammonia slip condition.

FIG. 3 shows a flow chart illustrating a routine for detecting ammoniaslip in an exhaust gas treatment system.

FIG. 4 shows a flow chart illustrating a routine for controllingoperating parameters when an exhaust gas sensor output is allocated tonitrogen oxide.

FIG. 5 shows a flow chart illustrating a routine for controllingoperating parameters when an exhaust gas sensor output is allocated toammonia.

DETAILED DESCRIPTION

The following description relates to methods and systems for detectingNH₃ slip from an SCR system based on transient NOx signals detectedtherein. In one example, a method comprising using information from twoNOx sensors, a feedgas sensor located upstream of the SCR and a tailpipesensor located downstream, to predict a tailpipe NOx slope responsive tothe transient feedgas NOx signal is described. The method furthercomprises generating an envelope around the expected tailpipe NOx signaland allocating output from the NOx sensor to each of ammonia andnitrogen oxide in different amounts depending on changes in sensoroutput. For example, a transient tailpipe sensor output that fallsoutside of an expected envelope indicates an exhaust system that isslipping NH₃, which is further quantified by ramping a counter positivetoward an upper level that indicates NH₃ slip. Conversely, a transienttailpipe sensor output falling within the expected envelope indicatesNOx slip, which is further quantified by ramping a counter negativetoward a lower level that indicates NOx slip. In this manner, theexhaust gas sensor may be used to indicate both a reduced exhaust gastreatment system efficiency and an NH₃ slip condition. The methodfurther comprises adjusting one or more operating parameters based onthe allocation and change in sensor output.

Referring now to FIG. 1, a schematic diagram showing one cylinder ofmulti-cylinder engine 10, which may be included in a propulsion systemof an automobile, is illustrated. The engine 10 may be controlled atleast partially by a control system including a controller 12 and byinput from a vehicle operator 132 via an input device 130. In thisexample, input device 130 includes an accelerator pedal and a pedalposition sensor 134 for generating a proportional pedal position signalPP. A combustion chamber (or cylinder) 30 of the engine 10 may includecombustion chamber walls 32 with a piston 36 positioned therein. Thepiston 36 may be coupled to a crankshaft 40 so that reciprocating motionof the piston is translated into rotational motion of the crankshaft.The crankshaft 40 may be coupled to at least one drive wheel of avehicle via an intermediate transmission system. Further, a startermotor may be coupled to the crankshaft 40 via a flywheel to enable astarting operation of the engine 10.

The combustion chamber 30 may receive intake air from an intake manifold44 via an intake passage 42 and may exhaust combustion gases via anexhaust passage 48. The intake manifold 44 and the exhaust passage 48can selectively communicate with the combustion chamber 30 viarespective intake valve 52 and exhaust valve 54. In some embodiments,the combustion chamber 30 may include two or more intake valves and/ortwo or more exhaust valves.

In the example depicted in FIG. 1, the intake valve 52 and exhaust valve54 may be controlled by cam actuation via respective cam actuationsystems 51 and 53. The cam actuation systems 51 and 53 may each includeone or more cams and may utilize one or more of cam profile switching(CPS), variable cam timing (VCT), variable valve timing (VVT), and/orvariable valve lift (VVL) systems that may be operated by the controller12 to vary valve operation. The position of the intake valve 52 and theexhaust valve 54 may be determined by position sensors 55 and 57,respectively. In alternative embodiments, the intake valve 52 and/orexhaust valve 54 may be controlled by electric valve actuation. Forexample, the cylinder 30 may alternatively include an intake valvecontrolled via electric valve actuation and an exhaust valve controlledvia cam actuation including CPS and/or VCT systems.

In some embodiments, each cylinder of the engine 10 may be configuredwith one or more fuel injectors for providing fuel thereto. As anon-limiting example, the cylinder 30 is shown including one fuelinjector 66. The fuel injector 66 is shown coupled directly to thecylinder 30 for injecting fuel directly therein in proportion to thepulse width of signal FPW received from the controller 12 via anelectronic driver 68. In this manner, the fuel injector 66 provides whatis known as direct injection (hereafter also referred to as “DI”) offuel into the combustion cylinder 30.

It will be appreciated that in an alternate embodiment, the injector 66may be a port injector providing fuel into the intake port upstream ofthe cylinder 30. It will also be appreciated that the cylinder 30 mayreceive fuel from a plurality of injectors, such as a plurality of portinjectors, a plurality of direct injectors, or a combination thereof.

In one example, the engine 10 is a diesel engine that combusts air anddiesel fuel through compression ignition. In other non-limitingembodiments, the engine 10 may combust a different fuel includinggasoline, biodiesel, or an alcohol containing fuel blend (e.g., gasolineand ethanol or gasoline and methanol) through compression ignitionand/or spark ignition.

The intake passage 42 may include a throttle 62 having a throttle plate64. In this particular example, the position of the throttle plate 64may be varied by the controller 12 via a signal provided to an electricmotor or actuator included with the throttle 62, a configuration that iscommonly referred to as electronic throttle control (ETC). In thismanner, the throttle 62 may be operated to vary the intake air providedto the combustion chamber 30 among other engine cylinders. The positionof the throttle plate 64 may be provided to the controller 12 bythrottle position signal TP. The intake passage 42 may include a massair flow sensor 120 and a manifold air pressure sensor 122 for providingrespective signals MAF and MAP to the controller 12.

Further, in the disclosed embodiments, an exhaust gas recirculation(EGR) system may route a desired portion of exhaust gas from the exhaustpassage 48 to the intake passage 42 via an EGR passage 140. The amountof EGR provided to the intake manifold 44 may be varied by a controller12 via an EGR valve 142. By introducing exhaust gas to the engine, theamount of available oxygen for combustion is decreased, thereby reducingcombustion flame temperatures and reducing the formation of NO_(x) forexample. As depicted, the EGR system further includes an EGR sensor 144which may be arranged within the EGR passage 140 and may provide anindication of one or more of pressure, temperature, and concentration ofthe exhaust gas. Under some conditions, the EGR system may be used toregulate the temperature of the air and fuel mixture within thecombustion chamber, thus providing a method of controlling the timing ofignition during some combustion modes. Further, during some conditions,a portion of combustion gases may be retained or trapped in thecombustion chamber by controlling exhaust valve timing, such as bycontrolling a variable valve timing mechanism.

An exhaust system 128 includes an exhaust gas sensor 126 coupled to theexhaust passage 48 upstream of an exhaust gas treatment system 150. Thesensor 126 may be any suitable sensor for providing an indication ofexhaust gas air/fuel ratio such as a linear oxygen sensor or UEGO(universal or wide-range exhaust gas oxygen), a two-state oxygen sensoror EGO, a HEGO (heated EGO), a NO_(x), HC, or CO sensor. The exhaust gastreatment system 150 is shown arranged along the exhaust passage 48downstream of the exhaust gas sensor 126.

In the example shown in FIG. 1, the exhaust gas treatment system 150 isa urea based selective catalytic reduction (SCR) system. The SCR systemincludes at least an SCR catalyst 152, a urea storage reservoir 154, anda urea injector 156, for example. In other embodiments, the exhaust gastreatment system 150 may additionally or alternatively include othercomponents, such as a particulate filter, lean NO_(x) trap, three waycatalyst, various other emission control devices, or combinationsthereof. In the depicted example, the urea injector 156 provides ureafrom the urea storage reservoir 154. However, various alternativeapproaches may be used, such as solid urea pellets that generate anammonia vapor, which is then injected or metered to the SCR catalyst152. In still another example, a lean NO_(x) trap may be positionedupstream of SCR catalyst 152 to generate NH₃ for the SCR catalyst 152,depending on the degree or richness of the air-fuel ratio fed to thelean NO_(x) trap.

The exhaust gas treatment system 150 further includes an exhaust gassensor 158 positioned downstream of the SCR catalyst 152. In thedepicted embodiment, the exhaust gas sensor 158 may be a NO_(x) sensor,for example, for measuring an amount of post-SCR NO_(x). In someexamples, an efficiency of the SCR system may be determined based on theexhaust gas sensor 158, for example, and further based on the exhaustgas sensor 126 (when the sensor 126 measures NO_(x), for example)positioned upstream of the SCR system . In other examples, the exhaustgas sensor 158 may be any suitable sensor for determining an exhaust gasconstituent concentration, such as a UEGO, EGO, HEGO, HC, CO sensor,etc.

The controller 12 is shown in FIG. 1 as a microcomputer, including amicroprocessor unit 102, input/output ports 104, an electronic storagemedium for executable programs and calibration values shown as a readonly memory chip 106 in this particular example, random access memory108, keep alive memory 110, and a data bus. The controller 12 may be incommunication with and, therefore, receive various signals from sensorscoupled to the engine 10, in addition to those signals previouslydiscussed, including measurement of inducted mass air flow (MAF) fromthe mass air flow sensor 120; engine coolant temperature (ECT) from atemperature sensor 112 coupled to a cooling sleeve 114; a profileignition pickup signal (PIP) from a Hall effect sensor 118 (or othertype) coupled to the crankshaft 40; throttle position (TP) from athrottle position sensor; absolute manifold pressure signal, MAP, fromthe sensor 122; and exhaust constituent concentration from the exhaustgas sensors 126 and 158. Engine speed signal, RPM, may be generated bycontroller 12 from signal PIP.

The storage medium read-only memory 106 can be programmed withnon-transitory, computer readable data representing instructionsexecutable by the processor 102 for performing the methods describedbelow as well as other variants that are anticipated but notspecifically listed.

In one example, the controller 12 may detect NH₃ slip based on outputfrom the exhaust gas sensor 158, as will be described in greater detailbelow with reference to FIG. 2. As an example, when the sensor 158detects a threshold increase in NO_(x) output, the controller 12 adjuststhe EGR valve 142 to reduce an amount of EGR such that NO_(x) emissionfrom the engine 10 increases. Based on the change in sensor outputduring the period of reduced EGR, the sensor output is allocated toNO_(x) or NH₃. For example, if the sensor output increases, the outputis allocated to NO_(x), as increased NO_(x) from the engine is notreduced by the SCR system. On the other hand, if the sensor output doesnot change by more than a threshold amount, the output is allocated toNH₃ and NH₃ slip is indicated. Based on the change in output and theallocation, the controller 12 may adjust one or more engine operatingparameters. As non-limiting examples, the controller 12 may adjust theamount of EGR and/or the delivery of reductant based on the change inoutput and the allocation.

As described above, FIG. 1 shows one cylinder of a multi-cylinderengine, and each cylinder may similarly include its own set ofintake/exhaust valves, fuel injector, spark plug, etc.

Turning to the plots shown in FIGS. 2A-D, an example transient NOxsignal depicts an ammonia slip condition is shown for the two sensorsystem of FIG. 1. Because NOx sensors produce output signals responsiveto both NOx and NH₃, a method to detect NH₃ slip may be useful formanaging the output of the exhaust system and resources therein. Forexample, if an SCR system is loaded with urea to the point ofsaturation, which varies with temperature, it may start to slip NH₃. TheNH₃ slipping past the SCR may be read by the tailpipe NOx sensor as NOx,which confuses the SCR control and monitoring system into thinking thesystem has a lower efficiency than it really has since some of thesignal is actually due to NH₃.

In FIGS. 2A-D, four temporal plots are shown that exemplify the method.The four plots are related and therefore use the same time axis, whichfor simplicity is shown along the bottom plot. Furthermore, although thedata is shown schematically as a function of time in seconds, the unitof time is not limiting and other units of time are possible. From topto bottom, the four plots represent: NOx signals collected by NOxsensors in the exhaust gas system; derivative plots of the predicted andactual slopes of the tailpipe NOx sensor according to the method; a plotshowing the difference between predicted and actual slopes of thetailpipe NOx sensor; and a counter with a threshold to indicate NH₃slip.

In FIG. 2A, an example feedgas signal 202 is shown dashed and an exampletailpipe signal 204 is shown solid. When exhaust system 128 is in astate of NOx slip, for example, when the SCR is not saturated and NH₃ isnot being released into the exhaust system, the tailpipe signal maygenerally be proportional to the feedgas signal. As such, the feedgasNOx signal and tailpipe NOx signal may be in phase and follow each otherclosely. Furthermore, when the NOx conversion efficiency issubstantially zero, tailpipe signal 204 and feedgas signal 202 may besubstantially identical. Conversely, for higher NOx conversionefficiencies, the shape of tailpipe signal 204 may resemble the shape offeedgas signal 202 but be a scaled down version of the feedgas signal.Alternatively, when exhaust system 128 is in a state of NH₃ slip,tailpipe signal 204 may have a somewhat flattened appearance or undulateat a lower frequency than feedgas signal 202. Because of this, duringNH₃ slip there is usually a period of time where the two signals are outof phase. Although the tailpipe signal can exceed the feedgas signal,particularly after an increase in temperature, this generally happensduring transient or changing conditions, which allows NH₃ slip to beidentified from the two signals by the method described herein.

The method relies on a transient response of the NOx sensors in order toallocate signal to NH₃ and NOx. Therefore, a central feature of themethod is the rate of change of the NOx signal as a function of time, ord(NOx)/dt. FIG. 2B shows a derivative plot wherein the slope or rate ofchange of tailpipe signal 204 in FIG. 2A is plotted versus time.Transient NH₃ detection is built around a comparison of the actualtailpipe NOx slope detected to the predicted tailpipe NOx slopeexpected. As such, FIG. 2B includes actual slope 210 that represents therate of change of the tailpipe signal 204 from FIG. 2A. Actual slope 210is shown in four parts labeled a-d for reasons that will be described inmore detail below. The method further includes predicting a tailpipe NOxslope using the slope of the feedgas NOx (from feedgas signal 202 inFIG. 2A) and the current tailpipe NOx signal, or an instantaneousreading from the tailpipe sensor. The predicted slope 212 for tailpipeNOx sensor, for example, sensor 158 in FIG. 1, can be generated using aknown relationship. Herein, the tailpipe NOx slope is predicted using:(dTP_(NOx) /dt)_(exp)=(TP/FG)*(dFG_(NOx) /dt)_(act),where (dTP_(NOx)/dt)_(exp) is the expected or predicted rate of changeof the tailpipe signal, TP is an instantaneous tailpipe reading, FG isan instantaneous feedgas reading, and (dFG_(NOx)/dt)_(act) is the actualrate of change of the feedgas signal. Using this method, a comparison ofthe two slope signals based on the transient responses of the NOxsensors allows a high level of NH₃ detection sensitivity. For example,in some embodiments, the transient detection method can detect NH₃levels as low as 25 ppm.

To gauge how close actual slope 210 is to predicted slope 212 duringoperation, in other words, how the change in NOx signal detectedcorresponds to the change expected from feedgas signals and systemefficiencies, the ammonia slip detection (ASD) method described includesgenerating an envelope around the predicted slope curve. The envelopedefines a region around the predicted rate of change where the NOxoutput signal is likely to fall when the system is in NOx slip.Therefore, FIG. 2B shows two dash-dot lines that represent a positiveenvelope 214 offset from the predicted slope in the positive directionand negative envelope 216 offset from the predicted slope in thenegative direction. When taken together, both the positive and negativeenvelopes define a region around the predicted rate of change curve thatallows for signal discrimination and assessment of the NOx and NH₃levels in the exhaust system.

Returning to actual slope 210 that is shown in four parts labeled a-d.The different regions of the curve signify time periods when entryconditions are met such that a comparison between the two slope curvescan be expected to provide accurate determinations of the NOx and NH₃levels in the exhaust system. For example, sensors 126 and 158 withinexhaust system 128 are coupled to controller 12 that may includenon-transitory, computer readable data representing instructionsexecutable by processor 102 for enabling and disabling the method basedon operating conditions of the engine. Therefore, curves 210a and 210care shown as unbolded, dashed line segments to represent exemplaryperiods where entry conditions are not met and the method is disabled.Conversely, curves 210 b and 210 d are shown as bold, dashed linesegments to represent exemplary periods where entry conditions are metand the method is enabled. When engaged, a controller processes the databy comparing actual slope 210 to predicted slope 212 and the surroundingenvelope. Basically, when actual slope 210 falls within the envelope, awindow counter ramps negative and is decremented toward zero to indicatethe exhaust system is comprised of NOx whereas when actual slope 210falls outside of the envelope, the window counter ramps positive and isincremented toward an upper level away from zero to indicate thepresence of NH₃ slip in the exhaust system.

In some embodiments, during conditions of NH₃ slip the exhaust systemmay include a tailpipe NOx sensor signal comprised of lower frequencycontent relative to the feedgas signal. Because of this, an upper leveltailpipe frequency may be indicative of NH₃ slip. Therefore, when actualslope 210 is greater than a frequency threshold, high frequency contentmay be indicated that is interpreted as a NOx signal. In response, thewindow counter may be decremented toward zero to indicate NOx slipregardless of whether the slope falls inside or outside of the envelope.For example, in some embodiments, a rate of change,(dTP_(NOx)/dt)_(actual) (actual slope 210 in FIG. 2B), greater than amaximum allowed rate may be treated as a NOx response by the system.

In FIG. 2C, difference plot 220 is shown that reflects the relativedifference between actual slope 210 and predicted slope 212 from FIG.2B. For clarity, a horizontal line at y=0 that indicates no difference,is also shown. Therein, the fluctuations of the actual slope relative tothe predicted slope may be more clearly observed. For example, at earlytimes on the left a negative peak is observed that reflects a loweractual slope than was predicted by the method (e.g. actual slope 210 isless than predicted slope 212). Thereafter, following the contour of thedifference plot, the actual slope fluctuates around the predicted slopebased on conditions in the exhaust system. Although not shown, in someembodiments, other horizontal threshold lines may also be included tofurther indicate places where differences between the two plots aresubstantially large.

Turning to FIG. 2D, a plot of the window counter that is used forindicating NH₃ slip is shown. As described briefly above, when the ASDsystem is enabled by controller 12, the window counter increments towardan upper level that indicates NH₃ slip when actual slope 210 fallsoutside of the envelope, and decrements toward a lower level thatindicates NOx slip when actual slope 210 falls within the envelope.Therefore, window counter 230 is shown increasing when actual slope 210b falls outside of the envelope. In FIG. 2D, two thresholds are shown.First threshold 236 indicates NH₃ slip in the exhaust system. As such,when the window counter exceeds first threshold 236, an NH₃ flag is setto indicate that NH₃ is slipping from the SCR. For simplicity, in thisexample method, the NH₃ flag is a binary flag. Therefore, when windowcounter 230 is greater than first threshold 236, an NH₃ flag is setto 1. Alternatively, when window counter 230 falls below first threshold236, the NH₃ flag is reset to 0. In the example signal processingapplication shown, the detection system is enabled in the two regionsidentified at 210 b and 210 d.During these periods, the counter isactive and the controller uses the state of the system to identifywhether NH₃ slip is occurring or not. In some embodiments, the relativemagnitude of window counter 230 compared to first threshold 236 may beused to indicate when exhaust system 128 is slipping NH₃ while in otherembodiments, the instantaneous location of window counter 230 relativeto an upper level (indicating NH₃) and a lower level (e.g. 0 indicatingNOx) may be used to indicate a probability or degree of NH₃ slip in theexhaust system. In still other embodiments, a second threshold 234 maybe included that is lower than first threshold 236. When secondthreshold 234 is present, the NH₃ flag may be reset to 0 when windowcounter 230 falls below second threshold 234 instead of first threshold236, as was described above. Different thresholds allow for hysteresisin the system so the NH₃ flag is not reset to indicate NOx if windowcounter 230 falls briefly below first threshold 236. Rather, NOx isindicated when window counter 230 falls below the lower threshold thatis set to indicate a higher degree of NOx slip in the exhaust system.

Because the ammonia slip detection system is under the control ofcontroller 12, instructions for disabling the detection system may beincluded in the programmable software stored by the control system.Although the detection system can be disabled based on many conceivableoperating conditions, and many combinations of variables are possible,in one embodiment, the programmable instructions may implement thefollowing conditions to disable the detection system: a low SCRtemperature, high feedgas NOx levels indicating a saturated feedgassensor output, high tailpipe NOx levels indicating a saturated tailpipesensor output, low feedgas or tailpipe NOx levels below a detectionthreshold, high or low rates of change in the NOx conversion efficiency,low torque output by the engine system, low injection pulses of ureafrom a storage reservoir, a calibrateable delay after a feedgas sensoror tailpipe sensor becomes activate, a high rate of change of spacevelocity, a low exhaust flow, a minimum/maximum actual or predictedslope indicating a deadzone in the detected signal, and a low rate ofchange of feedgas NOx that identifies feedgas inflection points. Inresponse to detection of one or more of these conditions by controller12, the ASD method may be disabled so no processing of the signal occursin the manner described herein. For example, actual slope 210 c refersto a slope signal acquired during a period when the detection system isdisabled. As another example, line 232 is a binary line indicating thedisable state of the system. Therefore, when line 232 is substantiallyon the x-axis, the ASD system is enabled and controller 12 may monitorthe exhaust conditions in the manner already described. Conversely, whenline 232 is above the x-axis, the ASD system may be disabled so nosignal processing occurs. As such, further processing of the tailpipeNOx signal is substantially prohibited since the information obtainedmay not reliably express NOx and NH₃ levels within the exhaust system.During periods where the detection system is disabled, the controlsystem may still monitor conditions within the exhaust system andfurther have the flexibility to activate the detection system, which insome instances, may involve overriding the disabling software orconditions identified therein.

Turning to the method for processing NOx signals by the control system,in FIG. 3, a flow chart illustrating example method 300 for thedetection of ammonia slip in an exhaust gas treatment system is shown.Therein, the set of programmable decisions a controller may utilize whenallocating a NOx sensor signal to either NOx or NH₃, or a combinationthereof, is described.

At 302, method 300 includes determining the engine operating conditions.The operating conditions may include both engine operating conditions(e.g., engine speed, engine load, amount of EGR, air fuel ratio, etc.)and exhaust gas treatment system conditions (e.g., exhaust temperature,SCR catalyst temperature, amount of urea injection, etc.).

At 304, method 300 includes determining a predicted rate of change ofthe tailpipe NOx sensor and generating an envelope based on the expectedslope. As described above, the rate of change of the tailpipe NOx sensormay be predicted using the feedgas NOx sensor signal output and acurrent measure of the tailpipe NOx sensor signal output. Then, based onthe rate of change of the tailpipe NOx sensor predicted, the method mayfurther generate the envelope to define a region wherein the signal maybe expected to fall when the exhaust system is operating in conditionsof NOx slip. Although many methods can be conceived of to generate anenvelope, in some embodiments, the envelope is simply a percentage ofthe predicted slope that is offset from the predicted slope in thepositive and negative directions. For example, a controller that definesa region within 5% of a predicted slope of 10.0 may generate a positiveenvelope with a value of 10.5 and a negative envelope with a value of9.5. Alternatively, if the predicted slope is smaller, e.g. 1.0, thepositive envelope may have a value of 1.05 and the negative envelope mayhave a value of 0.95. In this way, the envelope will define a regionsurrounding the predicted curve that is within 5% of the curve.Returning to the envelope of FIG. 2B, the size of the region defined bythe envelope clearly deviates as the magnitude of the slope of thepredicted curve fluctuates around zero. At 306, method 300 includesdetermining the actual rate of change of the tailpipe NOx sensor.

Although method 300 may monitor NOx sensors frequently, or evencontinuously, controller 12 may also enable or disable the system in themanner already described with respect to FIG. 2B. As such, at 308 method300 includes determining whether the entry conditions have been met. Ifcontroller 12 determines that the entry conditions allow for accuratemeasurements to be made by the detection system, for instance becausethe temperature of the SCR is above a threshold, then the ASD system maybe activated. Therefore, at 310, the activated system includes enablingthe window counter in order to compare the actual slope to the predictedslope, as indicated at 312. Alternatively, if controller 12 determinesthat an accurate measurement by the NOx system is not possible based onconditions detected in the engine system, at 314, the control system maydisable the counter so no further signal processing occurs after signalacquisition. In some embodiments, when the ASD system is disabled, thecounter may be reset by ramping the counter negative to indicate NOxslip by the system. In other embodiments, the counter may not be rampedin the manner described above, but simply hold a value until thedetection system is reactivated.

Returning to 312, wherein controller 12 has determined that the entryconditions have been met and the detection system is activated to allowadjustment of a counter based on a comparison between the actual andpredicted NOx rates, once the comparison is made, at 318 the controllermay be programmed to determine whether the actual slope falls within theenvelope. Then, based on a location of the actual slope relative to theenvelope, a positive or negative score may be assigned based on therelative differences. As described in more detail above with respect toFIG. 2D, at 320 the counter is ramped positive toward an upper levelthat indicates NH₃ slip when the actual slope falls outside of theenvelope whereas at 316 the counter is ramped negative toward a lowerlevel (e.g. zero) indicating NOx slip when the measured rate fallswithin the envelope.

After ramping the counter based on the relative location of the actualslope compared to the envelope surrounding the predicted rate of change,at 322 method 300 further compares the counter to a threshold todetermine whether the tailpipe NOx sensor is to be allocated to NOx orNH₃. In one embodiment, sensor allocation includes allocating a firstportion of the NOx sensor output to NOx and a second, remaining portionof the NOx sensor output to ammonia. Then, based on the allocation,delivering reductant to the engine exhaust based on each of the firstand second portions. For example, reductant can be increased responsiveto increased NOx but decreased responsive to increased NH₃. Therefore,the amount of reductant injected generally depends on the relativeamounts indicated by the first and the second portions.

If the counter is above the first threshold, for example first threshold236 in FIG. 2D, at 324 controller 12 may allocate at least some of thetailpipe output signal to NH₃ slip and set a flag to indicate such at326. Alternatively, if controller 12 determines that the counter fallsbelow the first threshold, at 328 it may allocate at least some of thetailpipe output signal to NOx slip and set a flag to indicate such at330. In some embodiments, the current status of the sensor allocationmay correspond to a probability of NH₃ slip while in other embodiments,NH₃ slip may be indicated by a binary flag. In this manner, controller12 can detect ammonia slip within the exhaust system and allocate theNOx sensor output to either one or both of NOx and NH₃ whilecommunicating the current status to a driver and adjusting one or moreoperating parameters based on the sensor output.

Continuing to FIG. 4, a routine for adjusting system operation based onthe allocation of sensor output to NO_(x) is shown. Specifically, theroutine determines an exhaust NO_(x) concentration downstream of the SCRcatalyst and adjusts one or more operating parameters based on thesensor output.

At 402, operating conditions are determined. As described above, theoperating conditions may include engine operating conditions (e.g.,engine speed, engine load, amount of EGR, air fuel ratio, etc.) andexhaust gas treatment system conditions (e.g., exhaust temperature, SCRcatalyst temperature, amount of urea injection, etc.).

Once the operating conditions are determined, the routine proceeds to404 and the exhaust NO_(x) concentration downstream of the SCR catalystis determined based on the exhaust gas sensor output.

At 406, one or more operating parameters are adjusted based on theNO_(x) concentration. As non-limiting examples, the operating parametersmay include amount of EGR and amount of urea injection, or dosing level,wherein the urea dosing level may be adjusted until an actual NOxefficiency matches a predicted NOx efficiency. For example, the amountof EGR may be increased by an amount corresponding to the change inNO_(x) amount above the threshold amount. By increasing the amount ofEGR, less NO_(x) may be emitted by the engine resulting in a reducedamount of NO_(x) passing through the SCR catalyst. As another example,the amount of urea injection may be increased by an amount correspondingto the change in NO_(x) amount above the threshold amount and atemperature of the SCR catalyst. The amount of urea injection may beincreased by changing the pulsewidth or duration of the urea injection,for example. By increasing the amount of urea injected to the SCRcatalyst, a greater amount of NO_(x) may be reduced by the catalyst,thereby reducing the amount of NO_(x) which passes through the catalyst.In other examples, a combination of amount of EGR and amount of ureainjection may be adjusted.

In FIG. 5, a routine for adjusting system operation based on theallocation of sensor output to NH₃ is shown. Specifically, the routinedetermines an exhaust NH₃ concentration downstream of the SCR catalystand adjusts one or more operating parameters based on the sensor output.

At 502, operating conditions are determined. As described above, theoperating conditions may include engine operating conditions (e.g.,engine speed, engine load, amount of EGR, air fuel ratio, etc.) andexhaust gas treatment system conditions (e.g., exhaust temperature, SCRcatalyst temperature, amount of urea injection, etc.).

Once the operating parameters are determined, the routine continues to504 and the exhaust NH₃ concentration downstream of the SCR catalyst isdetermined based on the exhaust sensor output.

At 506, one or more operating parameters are adjusted based on the NH₃concentration. As non-limiting examples, the operating parameters mayinclude amount of urea injection and amount of EGR. For example, theamount of urea injection may be reduced such that an amount of excessNH₃ which slips from the SCR catalyst is reduced. As described above,the amount of urea injection may be increased by changing the pulsewidthor duration of the urea injection. As another example, the amount of EGRmay be reduced. For example, by reducing the amount of EGR, a greateramount of NO_(x) may be emitted from the engine. The increased NO_(x)may be reduced by the excess NH₃ in the SCR catalyst, thereby reducingthe amount of NO_(x) which passes through the SCR catalyst.

With regard to urea dosing, in one embodiment, the exhaust system may bean adaptive SCR system that achieves the proper adaptive value byadjusting the urea dosing level until the actual NOx efficiency matchesthe predicted NOx efficiency. For example, as tailpipe NOx levelsincrease, the calculated NOx efficiency decreases. If the efficiencydrops too low, the adaptive system responds by increasing urea dosing toachieve the predicted NOx efficiency. Conversely, as NH₃ levelsincrease, the calculated efficiency also decreases since NH₃ looks likeNOx to a NOx sensor. As such, the adaptive system responds by decreasingurea dosing to achieve the predicted efficiency. Because the adaptivecorrection is different for NOx versus NH₃ slip, the control system maydepend on allocation of a NOx sensor output to NOx and NH₃ by themethods described herein.

The amount the operating parameters are adjusted may be further based ona temperature of the SCR catalyst, as the point of urea saturation ofthe catalyst varies with temperature. For example, when the temperatureof the catalyst is a relatively higher temperature, the amount of EGRmay be reduced less and/or the amount of urea injection may be reducedby a smaller amount. In contrast, when the temperature of the catalystis a relatively lower temperature, the amount of EGR may be increasedmore and/or the amount of urea injection may be reduced by a largeramount.

In other examples, only the amount of EGR may be decreased or only theamount of urea injected to the SCR catalyst may be increased. In stillother examples, one or more other operating parameters may beadditionally or alternatively adjusted. As such, one or more operatingparameters are adjusted in order to reduce the NH₃ slip.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The specific routines described herein may represent one or more of anynumber of processing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various acts,operations, or functions illustrated may be performed in the sequenceillustrated, in parallel, or in some cases omitted. Likewise, the orderof processing is not necessarily required to achieve the features andadvantages of the example embodiments described herein, but is providedfor ease of illustration and description. One or more of the illustratedacts or functions may be repeatedly performed depending on theparticular strategy being used. Further, the described acts maygraphically represent code to be programmed into the computer readablestorage medium in the engine control system.

This concludes the description. The reading of it by those skilled inthe art would bring to mind many alterations and modifications withoutdeparting from the spirit and the scope of the description. For example,I3, I4, I5, V6, V8, V10, and V12 engines operating in natural gas,gasoline, diesel, or alternative fuel configurations could use thepresent description to advantage.

The following claims particularly point out certain combinations andsubcombinations regarded as novel and nonobvious. These claims may referto “an” element or “a first” element or the equivalent thereof. Suchclaims should be understood to include incorporation of one or more suchelements, neither requiring nor excluding two or more such elements.Other combinations and subcombinations of the disclosed features,functions, elements, and/or properties may be claimed through amendmentof the present claims or through presentation of new claims in this or arelated application. Such claims, whether broader, narrower, equal, ordifferent in scope to the original claims, also are regarded as includedwithin the subject matter of the present disclosure.

The invention claimed is:
 1. A method for an engine comprising: allocating a NOx sensor output to each of NH₃ and NOx based on calculating upstream and downstream NOx rates of change relative to an SCR emission device, determining an expected downstream NOx rate of change, generating an envelope surrounding the expected downstream NOx rate of change, and comparing the calculated downstream NOx rate of change to the expected downstream NOx rate of change and the envelope surrounding the expected downstream NOx rate of change, where the expected downstream NOx rate of change is determined based on the calculated upstream rate of change; and adjusting reductant delivery into engine exhaust via an injector based on the allocation.
 2. The method of claim 1 wherein more of the NOx sensor output is allocated to NOx than NH₃ when the calculated downstream NOx rate of change is within the envelope.
 3. The method of claim 1 wherein a first portion of the NOx sensor output is allocated to NOx and a second, remaining portion of the NOx sensor output to NH₃, wherein reductant delivery is based on each of the first and second portions, with different adjustments for the first portion as compared to the second portion.
 4. The method of claim 1 wherein a portion of the NOx sensor output is allocated to NH₃ when the calculated downstream NOx rate of change is outside of the envelope.
 5. The method of claim 1 wherein the upstream NOx rate of change is determined based on a sensor.
 6. The method of claim 1 wherein the allocating is further based on a counter that quantifies the comparison of the calculated downstream NOx rate of change and the envelope.
 7. The method of claim 6 wherein the counter ramps negative to a lower level that indicates NOx slip when the calculated downstream NOx rate of change is within the envelope and ramps positive to an upper level that indicates NH₃ slip when the calculated downstream NOx rate of change is outside of the envelope.
 8. The method of claim 7 wherein the allocation is based on the counter relative to the lower level and upper level.
 9. The method of claim 8 wherein an additional operating parameter is adjusted based on the allocation.
 10. A method for performing real-time control of ammonia slip in an engine exhaust system, comprising: allocating an output of a NOx sensor arranged downstream of an SCR emission device to each of NH₃ and NOx based on: predicting a a rate of change of NOx downstream of the SCR emission device, generating an envelope that surrounds the predicted rate of change, and comparing an actual rate of change of NOx downstream of the SCR emission device to the predicted rate of change and the envelope, the actual rate of change based on the output from the NOx sensor downstream of the SCR emission device; and adjusting delivery of a reductant to engine exhaust upstream of the SCR emission device via an injector based on the allocation.
 11. The method of claim 10 further comprising, if the actual rate of change is outside of the envelope surrounding the predicted slope, determining there is an NH₃ slip condition.
 12. The method of claim 11 wherein the predicted rate of change is based on an upstream NOx rate of change.
 13. The method of claim 12 wherein the predicted rate of change is further based on a current output of the NOx sensor arranged downstream of the SCR emission device and a current output of a NOx sensor arranged upstream of the SCR emission device.
 14. The method of claim 10 wherein the allocation is further based on a counter that quantifies the comparison of the actual rate of change and the predicted rate of change.
 15. The method of claim 10 wherein the envelope surrounding the predicted rate of change is comprised of a positive envelope offset from the predicted rate of change in a positive direction, and a negative envelope offset from the predicted rate of change in a negative direction.
 16. A system, comprising: an engine with an exhaust system; an exhaust treatment system disposed in the exhaust system and including an SCR emission device, a urea injector disposed upstream of the SCR emission device, a NOx sensor disposed downstream of the SCR emission device, and a NOx sensor disposed upstream of the SCR emission device; and a control system in communication with the exhaust treatment system, where the control system includes non-transitory instructions for NH₃ slip detection based on a transient NOx signal, wherein the NH₃ slip detection includes: allocating an output of the NOx sensor disposed downstream of the SCR emission device to each of NH₃ and NOx and adjusting injection of reductant via the urea injector to engine exhaust gas upstream of the SCR emission device based on the allocation, where the allocation is based on: a comparison of an envelope which surrounds an expected rate of change of NOx downstream of the SCR emission device and a calculated rate of change of NOx downstream of the SCR emission device, wherein if the calculated rate of change of NOx downstream of the SCR emission device is within the envelope, determining there is a NOx slip condition. 